1. Field of Invention
The invention relates to an apparatus for temporarily supporting an offshore platform substructure or jacket on soft, unconsolidated ocean floors, and more particularly to a mudmat that is lightweight and corrosion resistant.
2. Description of the Prior Art
Currently, much of the hydrocarbons produced from the earth are extracted from beneath the ocean floor. Various types of structures have been employed in these offshore extraction operations. Typically, the structures consist of a horizontal working platform or equipment deck which is supported above the water's surface by a substructure, commonly referred to as an offshore jacket. Offshore jackets are most often fabricated onshore, towed or transported by barge to the drilling site, and lowered to the proper position on the sea floor.
Generally, an offshore jacket is comprised of at least three substantially vertical legs that are interconnected by framing or cross-bracing members to form a triangular or rectangular base, wherein a leg is disposed at each corner of the base. In its upright position, the jacket rest on the sea floor with the bottom of the legs resting on the sea floor or slightly penetrating into the soil. The jacket is secured to the sea floor with piles which are either driven through the legs or driven through sleeves attached to the legs.
In many areas of the world, the soil of the sea floor is unconsolidated and very soft resulting in very low allowable bearing pressures. These soft sea floors occur frequently near the mouths of large rivers that empty into the oceans. Sea beds in the world which exhibit high hydrocarbon content but are characterized by soft soils from river deltas include areas in the Gulf of Mexico, west Africa and southeast Asia.
The low bearing pressures of these unconsolidated sea floors create jacket support problems during installation of offshore platforms. Specifically, without adequate support, the legs of a jacket will sink into the sea floor, causing the jacket to either fall onto its side or settle lower than design specifications. In any case, jacket settling due to a soft sea floor can negatively effect the alignment of the jacket as it is positioned at the drilling site. In this same vein, difficulties often arise during pile driving operations, which are generally completed within one to two weeks of placing a jacket in position on the sea floor. As a pile is driven into the sea bed through a sleeve, the leg or portion of the jacket to which the sleeve is attached tends to sink into the soft mud under forces applied during the pile driving operation, thus effecting the overall alignment of the jacket.
One solution to the difficulties associated with unconsolidated sea floors is to provide a structure that spreads the downward forces applied to the jacket over a larger area of the sea floor. The most common structure for accomplishing this task is called a mudmat. A mudmat has a very large surface area that rests against the sea floor (as opposed to the comparatively small surface area of a jacket leg), distributing the load of the jacket over a larger sea floor, thus allowing the jacket to properly stand on the soft sea floor and to provide stability during pile-driving operations.
Mudmats are typically comprised of framing members which are attached to and provide support to a bearing plate. The bearing plate rests against the sea floor and provides the large surface area for force distribution. The mudmats themselves are attached to the bottom of a jacket, most often adjacent the legs of the jacket. Originally, bearing plates were fabricated from wood timbers with large amounts of steel support structure to back the bearing plates. These "wooden mudmats", however, are characterized by a number of drawbacks. The large, long timbers most suitable in fabricating such mudmats are often difficult to obtain and comparatively expensive. Such mudmats also require substantial amounts of man-hours to assemble and require large mounts of steel to provide the necessary backing support structure. Finally, although wooden mudmats provide some buoyancy in water, approximately 5-10 pounds per square foot, such mudmats are comparatively heavy in air, weighing approximately 30-40 pounds per square foot. The bulky nature of these prior art mudmats, i.e., large surface areas combined with comparatively large weights, render such mudmats difficult to manipulate and install.
One solution to the drawbacks associated with wooden mudmats has been to fabricate mudmat bearing plates out of stiffened steel plates, corrugated steel plates or steel sheet piles. These "steel mudmats" offer a number of improvements over wooden mudmats. Steel mudmats require less backing support structure than wooden mudmats. In addition, steel mudmats typically weigh less than wooden mudmats. Specifically, steel mudmats typically weigh in air approximately 22-30 pounds per square foot. However, steel mudmats have their own drawbacks. Steel mudmats are themselves comparatively heavy and are characterized by high fabrication costs. More significantly, steel mudmats are subject to high corrosion rates unless protected in some manner.
The functional life of mudmats is approximately the one to two weeks required for the pile-driving operations to be completed. After the installation of the piles, the mudmats become functionally useless for the remaining life of the offshore platform. However, offshore platforms are designed for a functional life of typically 10, 20, or 30 years, depending upon the development of the oil and gas field. Though the mudmats are functionally useless, steel mudmats are parasitic in nature in that they contribute to the drain of the cathodic protection that is provided for offshore platforms.
The cathodic protection is necessary to prevent oxidation and corrosion of the offshore platform and to prevent the subsequent reduction in its structural integrity. Aluminum-alloy ingots typically serve as the sacrificial anodes to protect the offshore platform. Since steel mudmats are generally attached to a jacket by welding to become part of the jacket structure, the mudmats are electrically connected to the offshore platform and contribute to the drain of the sacrificial anodes.
One solution to the problem of cathodic drain by the mudmats is to remove the mudmats from the jacket structure after pile-driving is complete. Typically, mudmat removal includes the use of divers who must be sent to the sea floor to cut the mudmats from the jacket. In addition, since the mudmats are generally bounded by permanent framing structure, the mudmats are extremely difficult to remove in one piece, and thus must be cut into smaller pieces that can be maneuvered around the permanent framing structure and lifted to the surface. This procedure is repeated over and over again for every piece of the mudmat until all pieces have been removed. Although effective, mudmat removal is undesirable because the procedure is costly and time consuming. Thus, there remains a need for mudmats that do not present a drain on the cathodic protection provided for the offshore platform itself, nor require removal following their useful life.
Turning back to the weights of both wooden and steel mudmats, offshore jackets are typically designed to have small amounts of reserve buoyancy, approximately 7-12% of the weight of the jacket, to permit ease in lifting, manipulation and positioning. The addition of heavy wooden or steel mudmats at the base of a jacket can negate this buoyancy and the beneficial effects realized by the buoyancy. To counter the weight of the mudmats, therefore, additional buoyancy must then be added to the top portions of the jacket. This is generally accomplished by providing larger diameter members for the legs and framing members. In so doing, however, not only is the overall cost of the jacket increased, but the susceptibility of the jacket to external wave forces is also increased. In other words, because of the small amounts of reserve buoyancy, offshore jackets are generally very sensitive to weight and buoyancy forces. As larger diameter members are incorporated into the structure, this sensitive balance is disrupted. Specifically, the larger diameter members provide a greater surface area against which ocean currents and waves can act. Not only can this require additional bracing to withstand these lateral forces, it can result in the need for enhanced pile support through either the addition of more piles or an increase in the depth to which piles are driven into the sea floor.
For the forgoing reasons, there remains a need for mudmats that will not adversely effect the weight and buoyancy of an offshore jacket to the degree of the prior art mudmats. The mudmats should avoid the need for cathodic protection or removal. In addition, the mudmats should exhibit lower fabrication costs than prior art mudmats. Finally, it would be desirable to provide mudmats that can be more easily fabricated and installed than prior art mudmats.